Details of a gamma-ray burst observed

An article published in the journal “Nature” describes the observation of a gamma-ray burst named GRB 160625B. An international team of astronomers led by Eleonora Troja of the University of Maryland used a number of telescopes after its discovery with NASA’s Fermi space telescope to detect the properties of this extremely energetic event, its geometry, the orientation of its jets and the origin of its extremely bright optical flash.

Gamma-ray bursts are extremely intense gamma rays emissions that can last only a few milliseconds but also many minutes. These are the most energetic events observable since the Big Bang is not observable, extreme objects are needed to trigger one such as black holes or at least neutron stars.

The gamma-ray burst detected by the Fermi space telescope was observed on June 25, 2016 and also by the MASTER-IAC (part of the Russian network of robotic MASTER telescopes) telescope and by the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA) radio telescopes. The various instruments’ sensitivity to very different ranges of the electromagnetic spectrum allowed to collect a lot of useful data on this phenomenon.

GRB 160625B took place at a distance of about 9 billion light years from Earth and was particularly brilliant even for gamma-ray bursts’ standards because a jet was aimed at the Earth. This allowed to study it in more detail than the average and to better understand its evolution, a process on which astrophysics still had many questions.

The origin of GRB 160625B has was attributed to the collapse of a very massive star, some tens of times the Sun’s mass, resulting in the birth of a black hole. The image shows the jets of particles ejected from the collapsed core at speeds close to that of light.

The collected data show first of all that the black hole generates a magnetic field so intense that it dominates the jets energy that form following the burst. Subsequently, the materials also part of the jets play a crucial role as well and this was the first surprise because in general astrophysicists thought that the jets were dominated by one of the two components while GRB 160625B’s observation show that both of them are important.

The observations suggest that synchrotron radiation, the result of electron acceleration on a curved or spiral trajectory, powers the first phase of the burst, called in jargon prompt phase. Other mechanisms were proposed but synchrotron radiation is the only one that can create the same kind of polarization and the same spectrum observed in the GRB 160625B burst.

Radiotelescopes are important to understand the various properties of the gamma-ray burst but for now there are few such events observed by these instruments. These events can have a very short duration so a quick reaction is needed to catch them and few radio observatories have this possibility. SKA, the new generation radio telescope which is currently being built, could be the solution.